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I know that keeping the trail leg strait during the swing is important to loading energy into the pole. But, it is later bent to help the vaulter get inverted.
When should this bending happen? Any other info about this part of the vault would also be hepful. Thanks.

i have trouble figuring out when each phase of the vault should happen, but to me it seems like i bend the trail leg maybe a second or two after the take off,,
But watching better vaulters jump it seems like its better to keep the trail leg straight the whole time,,,
i may be wrong,,,

there are two different styles of coming through the pole:
you can keep the trail leg strong and straight through the whole vault and just use your drive knee as a extra push. this way you are loading the pole more evenly. and keeping that swing leg strong will also help you complete your rockback.
or you can keep your trail leg straight through your take off and rebend the trail leg once you complete your rock back, then you proceed to push through both of your heels to drive your self up and off the top of the pole as you drop your shoulders.. if you aren't strong enough to get over your hand grip and push off the top of the pole, this will help you build that momentum.

I used to think that the trail leg shold be locked out straigth at all times, but have just reciently seen that for a proper petrov method you accualy bend it a little on take off so you can kick it hard like kicking a ball or something.. If you see a video of Tim Mack he really brings it back and kicks it hard, then it should pretty much stay straight until u are done with ur vault.

If your trail-leg is stiff after take-off, it will bend through the chord, simple as that. The biomechanics are above my head, so I'm trusting in greater wisdom here... But after that, the goal is to keep the leg completely extended, which not even Bubka did! One of the few flaws he had!